The invention generally relates to determining seawater conductivity for the compensation of electromagnetics-based survey data, such as data acquired by a controlled source electromagnetics survey, for example.
Geological formations that form a reservoir for the accumulation of hydrocarbons in the subsurface of the earth may be characterized by a network of interconnected paths. The network contains the hydrocarbons and allows the ingress of the hydrocarbons to and from the reservoir. Knowledge of the porosity, permeability of geological formations and nature of the pore fluids is typically sought to determine the behavior of the fluids in the network. From this information about porosity, permeability and pore fluid saturation, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the resistivity of geological formations is a function of both porosity and pore fluid type. Considering that hydrocarbons are electrically insulating and most water contains salts, which are highly conductive, resistivity measurements typically are a valuable tool in determining the presence of a hydrocarbon reservoir in the formations.
One innovation in marine hydrocarbon exploration is in the use of controlled source electromagnetics (CSEM). A CSEM survey typically involves towing a horizontal electric dipole source close to the sea floor. Electromagnetic receivers (formed from various combinations of electric field antennae and magnetic field sensors) are situated on the sea floor to detect the electric and magnetic fields that are generated by the electric dipole source so that the fields may be recorded. The recorded fields are subsequently analyzed for purposes of determining the parameters (such as formation resistivity ) of the surveyed geological formations.